1. Field of the Invention
The present invention relates to a planar illumination device, and more particularly to a planar illumination device for use as a backlight fitted on the back face of, for example, a liquid crystal display device.
2. Description of the Prior Art
In recent years, in the middle of a so-called IT (information technology) revolution, as dramatic developments are made in information communication fields, more and more stress has come to be placed on the portability of information terminal devices such as cellular telephones and personal computers. In keeping with this trend, also in liquid crystal display devices, more and more stress has come to be put on weight saving, thickness reduction, power saving, high and uniform brightness, and cost saving. This is true also with planar illumination devices such as backlights fitted on the back face of liquid crystal display devices.
First, as a conventional example, the planar illumination device disclosed in Japanese Patent Application Laid-Open No. H8-82714 will be described with reference to FIGS. 7A and 7B. A light source 12 is arranged along one side face of a light guide plate 11 having light-scattering particles dispersed in a transparent resin plate. Around the light source 12 is provided a lamp reflector 13 that reflects the light from the light source 12 and directs it into the light guide plate 11. Behind the light guide plate 11 is arranged a reflector plate 14 that scatteringly reflects light, and on the side faces of the light guide plate 11 other than that along which the light source 12 is provided are provided total-reflection members 15.
In this planar illumination device structured as described above, when the light source 12 is lit, the light therefrom is reflected by the lamp reflector 13 so as to enter the light guide plate 11, and is then uniformly reflected by the total-reflection members 15 at the side faces of the light guide plate 11. The light is then scatteringly reflected by the reflector plate 14, and this causes the top face of the light guide plate 11 to emit light. In this way, uniform light emission intensity is obtained within the light emission face 11c. 
Next, as another conventional example, the planar illumination device disclosed in Japanese Patent Application Laid-Open No. H11-24585 will be described with reference to FIG. 8. A light guide plate 21 and a fluorescent lamp 22 arranged along one side face of the light guide plate 21 are housed inside a case 23 that has an opening formed in the portion thereof facing the light emission face 21c of the light guide plate 21 and that has its inner surfaces so formed as to reflect light. Here, the light guide plate 21 is so processed that uniform brightness is obtained over its light emission face 21c. 
In this planar illumination device structured as described above, when the fluorescent lamp 22 is lit, the light therefrom is reflected by the case 23 so as to enter the light guide plate 21 from all directions through its side and bottom faces. Thus, the light is uniformly scattered, and this causes the top face of the light guide plate 21 to emit light. In this way, uniform light emission intensity is obtained within the light emission face 21c. 
However, the planar illumination device disclosed in Japanese Patent Application Laid-Open No. H8-82714 mentioned above requires additional provision of the total-reflection members 15 on the side faces of the light guide plate 11. This not only increases the total number of components, but also complicates the manufacturing process and thus increases the cost in cases where the light guide plate 11 is thin, because then the total-reflection members 15 need to be bonded thereto with an adhesive layer or the like interposed in between.
On the other hand, the planar illumination device disclosed in Japanese Patent Application Laid-Open No. H11-24585 mentioned above requires special processing of the light guide plate 21, such as dispersing light scattering particles therein, for the purpose of obtaining uniform brightness over the light emission face 21c thereof This, also, hampers cost reduction. Moreover, uses of this planar illumination device are limited by the fact that it has the case 23 outside.
An object of the present invention is to provide a highly versatile planar illumination device that offers uniform light emission intensity over its light emission face and that simultaneously permits further cost reduction.
To achieve the above object, according to one aspect of the present invention, a planar illumination device is provided with a light source that shines light into a light guide plate through at least one side face (i) thereof and a reflector member laid on the back face of the light guide plate opposite to the light emission face thereof Here, in the vicinity of at least one side face (ii) of the light guide plate other than the side face (i) thereof through which the light is shone into the light guide plate, a slit is formed so as to extend along that side face (ii). Moreover, a portion of the reflector member is, by being bent into an L shape, formed into an insertion portion, and this insertion portion is inserted into the slit.
In this arrangement, the insertion portion inserted into the slit functions as a reflector member at the side face (ii) of the light guide plate. Thus, the light emitted from the light source and shone into the light guide plate is first reflected by the insertion portion at the side face (ii), and is then reflected by the reflector member so as to travel from bottom to top, causing the entire light emission face to emit light with uniform light emission intensity.
The reflector member may have intermittent grooves formed therein to ease the bending of the insertion portion. This permits the insertion portion to be formed with ease and accuracy simply be being bent along the intermittent grooves.
According to another aspect of the present invention, a planar illumination device is provided with a light source that shines light into a light guide plate through one side face (i) thereof and a reflector member laid on the back face of the light guide plate opposite to the light emission face thereof Here, in the vicinity of another side face (ii) of the light guide plate opposite to the side face (i) thereof through which the light is shone into the light guide plate, a slit is formed so as to extend along that side face (ii). Moreover, a portion of the reflector member is, by being bent into an L shape, formed into an insertion portion, and this insertion portion is inserted into the slit.
In this arrangement, the insertion portion inserted into the slit functions as a reflector member at the side face (ii) of the light guide plate. Thus, the light emitted from the light source and shone into the light guide plate is first reflected by the insertion portion at the side face (ii), and is then reflected by the reflector member so as to travel from bottom to top, causing the entire light emission face to emit light with uniform light emission intensity.
The reflector member used in the present invention may be of any type as long as it reflects light. That is, an appropriate type of reflector member is selected so that it permits light to enter the light guide plate with optimum efficiency. For example, depending on whether the light source has directivity or not, an irregular-reflection member is used with a directional light source, and a regular-reflection member is used with a non-directional light source.
Moreover, in the present invention, as the light guide plate, a plate made of transparent resin is suitably used. For example, from among plates made of transparent resin such as acrylic resin, polycarbonate resin, or polyvinyl chloride resin, an appropriate one is selected to suit the intended use.
Moreover, to obtain as uniform brightness as possible within the light emission face of the light guide plate, any conventionally known method may be used, such as molding the light guide plate out of transparent resin and printing dots on the back face thereof opposite to the light emission face, or molding the light guide plate out of transparent resin and forming a large number of grooves or surface irregularities on the back face thereof opposite to the light emission face, or forming the light guide plate out of a light-scattering plate having light-scattering particles dispersed in transparent resin, or molding the light guide plate in the shape of a wedge having decreasing thicknesses from one side face, through which light is shone into it, to the opposite side face.
Moreover, the light source used in the present invention may be of any type. For example, an LED (light-emitting diode) or the like can be used.